Membrane separation processes are an increasingly important field in the art of separation science. They can be applied in the separation of a range of components of varying molecular weights in gas or liquid phases, including but not limited to nanofiltration, desalination and water treatment (see e.g. Basic Principles of Membrane Technology, Second Edition, M. Mulder, Kluwer Academic Press, Dordrecht. 564 p).
Membrane separation processes are widely applied in the filtration of aqueous fluids (e.g. desalination and waste water treatment). However, membrane separations have not been widely applied for the separation of solutes in organic solvents, despite the fact that organic filtrations, such as organic solvent nanofiltration has many potential applications in industry. This is mainly due to the relatively poor performance and/or stability of the membrances in organic solvents.
Many membranes for aqueous applications (e.g. desalination membranes, nanofiltration membranes) are thin film composite (TFC) membranes, which can be made by interfacial polymerisation (IFP). In the interfacial polymerisation technique, an aqueous solution of a reactive monomer (often a polyamine (e.g. a diamine)) is first deposited in the pores of a microporous support membrane (e.g. a polysulfone ultrafiltration membrane)—this step is also referred to as support membrane impregnation. Then, the porous support membrane loaded with the monomer is immersed in a water-immiscible (organic) solvent solution containing a second reactive monomer (e.g. a tri- or diacid chloride). The two monomers react at the interface of the two immiscible solvents, until a thin film presents a diffusion barrier and the reaction is completed to form a highly cross-linked thin film layer that remains attached to the support membrane. Since membranes synthesized via this technique usually have a very thin top layer, high solvent permeancies are expected.
The thin film layer can be from several tens of nanometres to several micrometres thick. The IFP technique is well known to those skilled in the art [Petersen, R. J. “Composite reverse osmosis and nanofiltration membranes”. J. Membr. Sci, 83, 81-150, 1993]. The thin film is selective between molecules, and this selective layer can be optimized for solute rejection and solvent flux by controlling the coating conditions and characteristics of the reactive monomers. The microporous support membrane can be selectively chosen for porosity, strength and solvent resistance.
A particularly preferred class of TFC membranes, well known in the art, are PA TFC membranes whereby polyamides are formed by interfacial polymerization on the surface of a porous support membrane.
U.S. Pat. No. 5,246,587 describes an aromatic polyimide RO membrane that is made by first coating a porous support material with an aqueous solution containing a polyamine reactant and an amine salt. Examples of suitable polyamine reactants provided include aromatic primary diamines (such as, m-phenylenediamine or p-phenylenediamine or substituted derivatives thereof, wherein the substituent is an alkyl group, an alkoxy group, a hydroxy alkyl group, a hydroxy group or a halogen atom; aromatic secondary diamines (such as, N,N-diphenylethylene diamine), cycloaliphatic primary diamines (such as cyclohexane diamine), cycloaliphatic secondary diamines (such as, piperazine or trimethylene dipiperidine); and xylene diamines (such as m-xylene diamine). The organic solution contains an amine-reactive polyfunctional acyl halide.
TFC membranes formed by IFP are often used for nanofiltration or reversed osmosis applications. Nanofiltration applications have gained attention based on the relatively low operating pressures, high fluxes and low operation and maintenance costs associated therewith. Nanofiltration is a membrane process utilising membranes of molecular weight cut-off in the range of 200-2,000 Daltons. Nanofiltration has been widely applied to filtration of aqueous fluids, but due to a lack of suitable solvent stable membranes, it has not been widely applied to the separation of solutes in organic solvents. This is despite the fact that organic solvent nanofiltration (OSN) has many potential applications in manufacturing industry including solvent exchange, catalyst recovery and recycling, purifications, and concentrations.
The support membranes generally used for commercial TFC membranes are often polysulfone or polyethersulfone ultrafiltration membranes. These supports have limited stability for organic solvents and, therefore, thin film composites membranes of the prior art which are fabricated with such supports cannot be effectively utilized for all organic solvent nanofiltration applications. WO2012010889 describes nanofiltration TFC composite membranes formed by IFP on a support membrane, made from e.g. crosslinked polyimide, wherein said support membrane is further impregnated with a conditioning agents and is stable in polar aprotic solvents. U.S. Pat. No. 5,173,191, suggests nylon, cellulose, polyester, Teflon and polypropylene as organic solvent resistant supports. U.S. Pat. No. 6,986,844 proposes the use of crosslinked polybenzimidazole for making suitable support membranes for TFC. However, there remains a need for solvent resistant membranes having good filtration properties (high permeancy & selectivity).
In the prior art, TFC membrane preparation by IFP comprises several steps: support membrane solidification (e.g. by phase inversion), support membrane saturation or impregnation by the amine monomer and the IFP reaction itself. In case of crosslinked PI support membranes (for use in e.g. solvent resistant applications) an additional crosslinking reaction step (with e.g. multifunctional amines) is required. These multiple steps make TFC membrane preparation by IFP a time-consuming (and hence uneconomic) process. There hence is a need in the art for improved TFC membrane preparation.